“If I run against her, I'd beat her. And they know it too. Hillary Clinton does not want to run against me,” he told ABC News' Jon Karl.

“They don't want to run against me," he added. "I guarantee you that if they had to choose who the Republican nominee is, it would not be me. And I know this. And you see it in their body language; you see it in the fact that they attack me more than anyone else."

Speaking to Karl on a bus ride between campaign events in New Hampshire, the third-place Iowa caucus finisher took swipes at that state’s winner, his fellow senator Ted Cruz, but he pulled his punches slightly with Donald Trump.

“My problem with Ted is he's very calculated. I mean, he's willing to say or do anything to get elected. He puts himself off as this sort of conservative purist, but that's not his record. Not on immigration or any other issue,” Rubio, who’s tussled with Cruz in several Republican debates, said.

He also condemned Cruz for sending an email to Iowa caucus supporters telling them Ben Carson was taking time off the campaign trail, while Carson had actually said he was stopping home for “fresh clothes” before continuing his campaign. Cruz has since apologized, and Carson accepted his apology.

“What they did was wrong,” Rubio said of the Cruz campaign. “I know that he's apologized and Ben has accepted his apology but I still think what happened to Ben was really unfair.”

About Trump, he said he’s had his “skirmishes with Donald,” but that most of his disagreements have to do with policy.

“Donald Trump has proven that he's capable of leading a large organization, his companies. And I don't think being president is the same as being a real estate developer. And on national security, he hasn't shown, to date, a deep level of depth on those issues. But that's what he's going to have to do now,” Rubio said.

But he added that some voters are drawn to the way Trump expresses himself.

“People like his plain-spokenness,” he said.

When the questions turned to his own campaign, Rubio former Sen. Rick Santorum, who endorsed Rubio after he dropped out of the race himself but who wasn’t able to name any of Rubio’s Senate achievements during a morning TV appearance.

“Rick just signed on to our campaign,” Rubio said. “That's unfair, to say someone has been your endorser for 12 hours and now he doesn't know your life history.”

But Rubio didn’t take the opportunity to name any of the accomplishments Santorum missed.

“But I'm proud of the things we've achieved. But my campaign's not about the past, about what we did. It's about what we're going to do too,” he said.